Induction of peroxisome proliferation in cultured hepatocytes by a series of halogenated acetates

New Perspectives for Cancer Hazard Evaluation by the Report on Carcinogens: A Case Study Using Read-Across Methods in the Evaluation of Haloacetic Acids Found as Water Disinfection By-Products

BACKGROUND

Due to the large number of chemicals not yet tested for carcinogenicity but to which people are exposed, the limited number of human and animal cancer studies conducted each year, and the frequent need for a timely response, mechanistic data are playing an increasingly important role in carcinogen hazard identification.

OBJECTIVES

To provide a targeted approach to identify relevant mechanistic data in our cancer evaluation of haloacetic acids (HAAs), we used several approaches including systematic review, the 10 key characteristics of carcinogens (KCs), and read-across methods. Our objective in this commentary is to discuss the strengths, limitations, and challenges of these approaches in a cancer hazard assessment.

METHODS

A cancer hazard assessment for 13 HAAs found as water disinfection by-products was conducted. Literature searches for mechanistic studies focused on the KCs and individual HAAs. Studies were screened for relevance and categorized by KCs and other relevant data, including chemical properties, toxicokinetics, and biological effects other than KCs. Mechanistic data were organized using the KCs, and strength of evidence was evaluated; this information informed potential modes of action (MOAs) and read-across-like approaches. Three read-across options were considered: evaluating HAAs as a class, as subclass(es), or as individual HAAs (analog approach).

DISCUSSION

Because of data limitations and uncertainties, listing as a class or subclass(es) was ruled out, and an analog approach was used. Two brominated HAAs were identified as target (untested) chemicals based on their metabolism and similarity to source (tested) chemicals. In addition, four HAAs with animal cancer data had sufficient evidence for potential listing in the Report on Carcinogens (RoC). This is the first time that the KCs and other relevant data, in combination with read-across principles, were used to support a recommendation to list chemicals in the RoC that did not have animal cancer data. https://doi.org/10.1289/EHP5672.

Evaluation of the role of peroxisome proliferator-activated receptor alpha (PPARalpha) in mouse liver tumor induction by trichloroethylene and metabolites

Trichloroethylene (TCE) is an industrial solvent and a widespread environmental contaminant. Induction of liver cancer in mice by TCE is thought to be mediated by two metabolites, dichloroacetate (DCA) and trichloroacetate (TCA), both of which are themselves mouse liver carcinogens. TCE, TCA, and DCA are relatively weak peroxisome proliferators (PP), a group of rodent hepatocarcinogens that activate a nuclear receptor, PP-activated receptor alpha (PPARalpha. The objective of this review is to assess the weight of evidence (WOE) that PPARalpha is or is not mechanistically involved in mouse liver tumor induction by TCE and metabolites. Based on similarities of TCE and TCA to typical PP, including dose-response characteristics showing PPARalpha-dependent responses coincident with liver tumor induction and abolishment of TCE and TCA effects in PPARalpha-null mice, the WOE supports the hypothesis that PPARalpha plays a dominant role in TCE- and TCA-induced hepatocarcinogenesis. Data indicates that the MOA for DCA tumor induction is PPARalpha-independent. Uncertainties remain regarding the genesis of the TCE-induced tumors. In contrast to the TCA-induced tumors, which have molecular features similar to those induced by typical PP, there is evidence, albeit weak, that TCE tumors arise by a mode of action (MOA) different from that of TCA tumors, based largely on dissimilarities in molecular markers found in TCE versus TCA-induced tumors. In summary, the WOE indicates that TCA-induced liver tumors arise by a PPARalpha-dependent MOA. Although the TCE MOA is likely dominated by a PPARalpha-dependent contribution from TCA, the contribution of a PPARalpha-independent MOA from DCA cannot be ruled out.

Dichloroacetate-induced modulation of cellular antioxidant enzyme activities and glutathione level in the J774A.1 cells

Dichloroacetate (DCA) is used for different medical and industrial purposes and has been found to be a toxic by-product produced during the process of water chlorination. The DCA effects on superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSH-Px) activities and glutathione (GSH) level were assessed and correlated with each other and also with cellular viabilities in J774A.1 macrophage cells. A concentration of 24 mm of DCA resulted in time-dependent decreases in cellular viability and glutathione level, and time-dependent increases in SOD activity when incubated with the cells for 24-48 h. DCA also resulted in significant increases in CAT and GSH-Px activities of the viable cells when incubated with the cells for 36 and 48 h. The changes in antioxidant enzyme activities and GSH levels were found to be strongly correlated with each other, and with cellular viabilities at different time points. While GSH did not result in any significant effects when added to the cells at concentrations ranging between 15 and 60 nmol ml(-1), it resulted in concentration-dependent increases in cellular viability when added to the DCA-treated cells, with maximal effects achieved at 45-60 nmol GSH ml(-1). However, cellular viability of the GSH + DCA treated cells remained below that of the control. Since viable cells from the DCA-treated cultures displayed significantly higher antioxidant enzyme activities compared with the control, it is concluded that those increases may have contributed to the cellular protection against DCA-induced cell death. Also, glutathione depletion has a major contribution to the observed cellular death induced by DCA.

Integrated disinfection by-products (DBP) mixtures research: gene expression alterations in primary rat hepatocyte cultures exposed to DBP mixtures formed by chlorination and ozonation/postchlorination

Large-scale differential gene expression analysis was used to examine the biological effects of disinfected surface waters on cultured rat hepatocytes. Source water from East Fork Lake (Harsha Lake), a reservoir on the Little Miami River in Ohio, was spiked with iodide and bromide and disinfected by chlorination or ozonation/postchlorination. The chlorinated and ozonated/postchlorinated waters were concentrated, respectively, 136- and 124-fold (full strength) by reverse-osmosis membrane techniques. Volatile disinfection by-products (DBP) lost during concentration were restored to the extent possible. Primary rat hepatocytes were exposed to either full-strength or 1:10 or 1:20 dilutions of the concentrates for 24 h and assayed for cytotoxicity and gene expression alterations. The full-strength concentrates were cytotoxic, whereas the diluted samples exhibited no detectable cytotoxicity. Differential gene expression analysis provided evidence for the underlying causes of the severe cytotoxicity observed in rat hepatocytes treated with the full-strength ozonation/postchlorination concentrate (e.g., cell cycle arrest, metabolic stasis, oxidative stress). Many gene expression responses were shared among the hepatocyte cultures treated with dilutions of the ozonation/ postchlorination and chlorination concentrates. The shift in the character of the response between the full-strength concentrates and the diluted samples indicated a threshold for toxicity. A small subset of gene expression changes was identified that was observed in the response of hepatocytes to peroxisome proliferators, phthalate esters, and haloacetic acids, suggesting a peroxisome proliferative response.

Lack of formic acid production in rat hepatocytes and human renal proximal tubule cells exposed to chloral hydrate or trichloroacetic acid

The industrial solvent trichloroethylene (TCE) and its major metabolites have been shown to cause formic aciduria in male rats. We have examined whether chloral hydrate (CH) and trichloroacetic acid (TCA), known metabolites of TCE, produce an increase in formic acid in vitro in cultures of rat hepatocytes or human renal proximal tubule cells (HRPTC). The metabolism and cytotoxicity of CH was also examined to establish that the cells were metabolically active and not compromised by toxicity. Rat hepatocytes and HRPTC were cultured in serum-free medium and then treated with 0.3-3mM CH for 3 days or 0.03-3mM CH for 10 days, respectively and formic acid production, metabolism to trichloroethanol (TCE-OH) and TCA and cytotoxicity determined. No increase in formic acid production in rat hepatocytes or HRPTC exposed to CH was observed over and above that due to chemical degradation, neither was formic acid production observed in rat hepatocytes exposed to TCA. HRPTC metabolized CH to TCE-OH and TCA with a 12-fold greater capacity to form TCE-OH versus TCA. Rat hepatocytes exhibited a 1.6-fold and three-fold greater capacity than HRPTC to form TCE-OH and TCA, respectively. CH and TCA were not cytotoxic to rat hepatocytes at concentrations up to 3mM/day for 3 days. With HRPTC, one sample showed no cytotoxicity to CH at concentrations up to 3mM/day for 10 days, while in another cytotoxicity was seen at 1mM/day for 3 days. In summary, increased formic acid production was not observed in rat hepatocytes or HRPTC exposed to TCE metabolites, suggesting that the in vivo response cannot be modelled in vitro. CH was toxic to HRPTC at millimolar concentrations/day over 10 days, while glutathione derived metabolites of TCE were toxic at micromolar concentrations/day over 10 days [Lock, E.A., Reed, C.J., 2006. Trichloroethylene: mechanisms of renal toxicity and renal cancer and relevance to risk assessment. Toxicol. Sci. 19, 313-331] supporting the view that glutathione derived metabolites are likely to be responsible for nephrotoxicity.

Modulation of the cytotoxicity and genotoxicity of the drinking water disinfection byproduct lodoacetic acid by suppressors of oxidative stress

Drinking water disinfection byproducts (DBPs) are generated by the chemical disinfection of water and may pose a hazard to the public health. Previously we demonstrated that iodoacetic acid was the most cytotoxic and genotoxic DBP analyzed in a mammalian cell system. Little is known of the mechanisms of its genotoxicity. The involvement of oxidative stress in the toxicity of iodoacetic acid was analyzed with the antioxidants catalase and butylated hydroxyanisole (BHA). lodoacetic acid toxicity was quantitatively measured with and without antioxidants in Salmonella typhimurium strain TA100 and with Chinese hamster ovary (CHO) cells. The endpoints included cytotoxicity in S. typhimurium or in CHO cells, mutagenicity in S. typhimurium, and genotoxicity in CHO cells. Neither catalase nor BHA reduced the level of iodoacetic acid induced cytotoxicity in S. typhimurium. In CHO cells neither antioxidant caused a significant reduction in iodoacetic acid induced cytotoxicity. However, in S. typhimurium, BHA or catalase reduced the mutagenicity of iodoacetic acid by 33.5 and 26.8%, respectively. Likewise, BHA or catalase reduced iodoacetic acid induced genomic DNA damage by 86.5 and 42%, respectively. These results support the hypothesis that oxidative stress is involved in the induction of genotoxicity and mutagenicity by iodoacetic acid.

Short-Term Chloral Hydrate Administration and Cancer in Humans

OBJECTIVE

Chloral hydrate, used as a hypnosedative in adults and children, has been shown to be genotoxic and carcinogenic in animal studies. We investigated the potential causal association between chloral hydrate exposure and cancer risk in humans.

METHODS

Cancer incidence was previously determined via biennial screening analyses of the 215 most commonly used drugs between 1976 and 1998 for a cohort of 143,574 outpatients at Kaiser Permanente who had prescriptions filled between 1969 and 1973. Among users of chloral hydrate, statistically significant elevations in standardised morbidity ratios were observed during various years for cancer at five anatomical sites, including the lung, stomach, prostate, skin melanoma and mouth floor. In this analysis, these associations were investigated using: (i) a dose-response analysis among exposed subjects; and (ii) a two-stage design with exposed and non-exposed persons.

RESULTS

There was evidence of an increasing risk of prostate cancer with increasing number of dispensings of chloral hydrate, which persisted after controlling for benign prostatic hypertrophy, vasectomy and obesity; however, the trend was not statistically significant. There was no evidence of a dose-response relationship between chloral hydrate and risk of any of the other four cancers. In the two-stage design, analyses comparing exposed and unexposed subjects showed no increased risk of cancer after controlling for confounding variables; however, the data were suggestive for prostate cancer, where the increased risk associated with chloral hydrate exposure after adjustment for confounding variables persisted. No dose-response relationship was seen for any of the other four cancer sites.

CONCLUSIONS

To our knowledge, this is the first study to examine the relationship between chloral hydrate exposure and cancer risk in humans. There was no persuasive evidence to support a causal relationship between chloral hydrate exposure in humans and the development of cancer. However, statistical power was low for weak associations, particularly for some of the individual cancer sites. Although animal data using much higher doses of chloral hydrate have demonstrated its genotoxicity and carcinogenicity, the effects of chloral hydrate in humans are still uncertain.